Fire control sprinkler systems generally include a plurality of individual sprinkler heads which are usually ceiling mounted about the area to be protected. The sprinkler heads are normally maintained in a closed condition and include a thermally responsive sensing member to determine when a fire condition has occurred. Upon actuation of the thermally responsive member the sprinkler head is opened, permitting pressurized water at each of the individual sprinkler heads to freely flow therethrough for extinguishing the fire. The individual sprinkler heads are spaced apart from each other, by distances determined by the type of protection they are intended to provide (e.g. light or ordinary hazard conditions) and the ratings of the individual sprinklers, as determined by industry accepted rating agencies such as Underwriters Laboratories, Inc., Factory Mutual Research Corp. and/or the National Fire Protection Association. It should be well appreciated that once the sprinkler heads have been thermally activated there should be minimal delay for the water flow through the sprinkler head at its maximum intended volume.
In order to minimize the delay between thermal actuation and proper dispensing of water by the sprinkler head, the piping that connects the sprinkler heads to the water source is, in many instances at all times filled with water. This is known as a wet system, with the water being immediately available at the sprinkler head upon its thermal actuation. However, there are many situations in which the sprinkler system is installed in an unheated area, such as warehouses. In those situations, if a wet system is used, and in particular since the water is not flowing within the piping system over long periods of time, there is a danger of the water within the pipes freezing. This will not only deleteriously affect the operation of the sprinkler system, should the sprinkler heads be thermally actuated while there may be ice blockage within the pipes, but such freezing, if extensive, can result in the bursting of the pipes, thereby destroying the sprinkler system. Accordingly, in those situations it is the conventional practice to have the piping devoid of any water during its non-activated condition. This is known as a dry fire protection system.
While all fire protection sprinkler systems generally include a check valve for isolating the sprinkler system piping from the pressurized water source during the non-activated condition, the design of such check valves for a dry type fire control sprinkler system has presented various problems. The check valve, which is interposed between the system piping and pressurized water source, includes a clapper, which when it is in its closed operative condition prevents the flow of the pressurized water into the sprinkler system piping. The sprinkler piping in the dry fire protection system will include air or some other inert gas (e.g. nitrogen) under pressure. The pressurized air, which is present within the sprinkler system piping, is also presented to the check valve. Should one or more of the sprinkler heads be thermally activated to its open condition, the pressure of the air within the sprinkler system piping and check valve will then drop. The check valve must be appropriately responsive to this drop pressure, normally in opposition to the system water pressure also present in the check valve, to move the clapper to its open condition. When this occurs, it is desirable to have a rapid expulsion of the pressurized air within the check valve and the sprinkler system piping, to permit the rapid flow of the pressurized water through the open check valve, into the sprinkler system piping, and through the individual sprinkler heads to rapidly extinguish the fire.
The check valves intended for dry type fire control sprinkler systems have typically controlled the clapper movement by the water and the air pressure applied to its opposite sides. Such fire check valves include an air seal which opposes the pressurized water seal. To appropriately apply the system air pressure over the surface of the clapper air seal, a priming water level is oftentimes maintained within the check valve. During normal conditions, when no sprinkler heads have been activated, the two seals will be an equilibrium, thereby maintaining the clapper in its closed condition.
In order to increase the speed of check valve operation upon a drop off of the system air pressure, occasioned by the activation of one or more sprinkler heads, the system air pressure is normally applied to the clapper air seal over a substantially greater area then the water pressure is applied to the clapper water seal. This is known as a high differential type check valve. A problem of such valves is that should there then be a reduction in the system water pressure after the clapper has opened, and particularly since the pressure against the opposite (air) side of the clapper has been increased with the column of water that has flowed therethrough, there is a tendency of the clapper to reclose. Since the pressure applied against the air seal of the clapper will now be increased by the column of water extending upwards from the reclosed check valve, a greater water pressure would now be required to move the clapper to its open condition. Such disadvantageous reclosure, is referred to as a water columning effect. This could result in failure of the check valve to subsequently open should one or more of the sprinkler heads be thermally activated.
In order to avoid the reclosure of the clapper such prior art dry system check valves have generally been provided with a mechanical latch to maintain the clapper in its open condition once it has been activated. The inclusion of such a mechanical latch, while serving to prevent reclosure, disadvantageously requires the entire sprinkler system to be shut down and the interior of the high differential type actuator accessed to release the latch and reclose the clapper after the fire has been extinguished. Thus prior dry system check valves have typically required the main supply of water to be shut off, the water drained from the system, and then the high differential check valve opened to manually unlatch and reset the clapper. Recognizing the disadvantage of having to manually access the interior of the check valve a mechanism is shown in U.S. Pat. Nos. 5,295,503 and 5,439,028 which includes a reset linkage mechanism attached to the check valve, and actuated by the rotation of an externally accessible handle. As can be well appreciated such a mechanism adds to the size, cost and complexity of the check valve.